1. Field of the Invention
The present invention relates to a transmitting apparatus and wireless communication method which can perform wireless communication by adaptive modulation (high-speed adaptive modulation).
2. Description of the Related Art
Recently, mobile stations such as a Personal Handy phone System (PHS), a portable telephone, and the like are widely used, and a telephone call or access to information becomes enabled irrespective of time and place. Nowadays, particularly, the amount of available information is ever increasing. In order to download large amount of data, a high-speed and high-quality wireless communication system has been employed.
As a standard for next-generation PHS communication capable of performing high-speed digital communication, for example, an Association of Radio Industries and Businesses (ARIB) STD T95 and PHS Memorandum of Understanding (MoU) are known. In such communication, an Orthogonal Frequency Division Multiplexing (OFDM) system is employed. The OFDM system is classified as one category of the multiplexing system and uses a plurality of carrier waves on a unit time axis, and frequency bands of the carrier waves are partly overlapped with one another so that, in adjacent carrier waves, the phases of signal waves to be modulated are orthogonal to each other, whereby the frequency band is effectively used.
In OFDM system, sub-channels are allocated to users by time sharing manner. Further, an Orthogonal Frequency Division Multiplexing Access (OFDMA) system is also provided in which a plurality of users share all sub-channels, and a sub-channel which is highest in transmission efficiency for each user is allocated to the user.
In ARIB STD T95 and PHS MoU, a receiving apparatus transmits a modulation and coding scheme (hereinafter, abbreviated as MCS) which is determined by adaptive modulation, to a transmitting apparatus through an anchor channel in a Fast access channel based on Map-Mode (FM-mode) (for example, see ARIB STD-T95), and the transmitting apparatus modulates data based on the MCS. Therefore, the transmitting apparatus and the receiving apparatus can perform communication using the MCS which is optimum in the communication environment at the timing.
In such adaptive modulation, based on the Signal to Interference and Noise Ratio (SINR) or bit error rate of a communication signal, the communication environment between the transmitting apparatus and the receiving apparatus is estimated, an MCS which exhibits a higher modulation efficiency is selected under a better communication environment, and that which exhibits a lower modulation efficiency is selected in a worse communication environment. Accordingly, stable wireless communication can be performed.
In ARIB STD T95 and PHS MoU, when a receiving apparatus receives erroneous data, the receiving apparatus transmits an Automatic Repeat reQuest (hereinafter, abbreviated as ARQ) which requests the transmitting apparatus which has performed the data transmission, to retransmit the data. In response to the ARQ, the transmitting apparatus performs the retransmission of the data by using a MAC layer (lower layer). Accordingly, the error can be efficiently compensated within a short control time (see A-GN4.00-01-TS Rev. 3 “Next Generation PHS Specifications”, Page 331-340).
Further, in ARIB STD T95 and PHS MoU, a Hybrid ARQ (HARQ) technique is employed in which the ARQ is combined with the forward error correction (FEC) to further improve the efficiency of packet error correction (for example, see JP-A-2006-502659 or WO2004/034656). Also a technique is known in which HARQ buffers are collectively erased in response to an affirmative response in such as HARQ (for example, see JP-A-2006-505219 or WO2004/042954).
In ARIB STD T95 and PHS MoU, when transmission data of the Quality of Service (QoS) class which does not accept delay, such as voice data which requires a real time property occur, a transmitting apparatus forcibly cancels (disable) transmission of retransmission data in response to an HARQ, and preferentially transmits the transmission data which does not accept delay. Therefore, the gain by the HARQ retransmission which expected to be obtained cannot be achieved. Despite this, assuming that the HARQ is enabled, the transmission data are transmitted while maintaining the MCS which is determined based on a request from the receiving apparatus, and which exhibits a high modulation efficiency. Therefore, an error may occur.